User blog:Samuli.seppanen/New little ladder, arch and washer rotation tool
During Christmas holidays I was at the forge making replacements for various ballista parts. As I mentioned earlier, my original little arch, and thus the little ladder, was too long, which caused all sorts of unnecessary fuss and misinterpretations. So those needed to be replaced. Here's a comparison of the length of old and new little ladder beams - the outer beams are the old ones: As can be seen, the old beams are way too long. The new beams are forged so that tenons' one side is in-line with the outer surface of the beam: This forging technique, when coupled with the notch attachment mechanism shown below, elegantly solves the problem of crosspieces and rungs being too close (3 dactyls) to each other and preventing insertion of the tenons into the pi-brackets. In my old little ladder I had to bend the beams outwards and the back inwards to work around this issue. The only other alternative would have been to make the crosspieces and rungs wider from the start. In addition to correcting the length of the little ladder and little arch, I decided to make both stronger, just in case. The old little ladder in particular was fairly thin, having been made from mere 4mm flat steel bar, so I decided to make the new one for 6mm steel unhardened spring steel. While both old and new tenons were made in exactly the same way, the increased thickness of the steel changed their nature significantly; first a new little ladder tenon from the side: And then an old one from side: The fade into the tenon in a new beam: And the same area in an old one: The increase in strength is quite obvious in a front view, despite the fact that the old little tenon has a wedge-shaped reinforcement welded to it: All in all, the new little ladder beams seem a lot more robust than its increased thickness alone would warrant for. The increase of total beam weight is reasonable, up from 1.1kg to 1.7kg. While the old little arch was probably almost as strong as needed, I overbuilt the new one a bit. The curved part is shallower, and the steel at the curve is thicker than elsewhere. The forks are now rectangular and much stronger than earlier. The weight of the old little arch was not measured, but was probably about 600 grams, whereas the new one weighs 748 grams. Here are a few pictures of the new little arch: I tried out several techniques for forging the little arch, spending probably more than a full working day without any visible progress. In the end I had to give up and forged all the parts separately and welded them together, which was trivial. The solution I ended up with is not as bad as it sounds, as the Romans used forge welding and could have assembled the little arch similarly. In my case the main problem was my forge, which has a deep fireplace, and did not work well for this kind of job. Forging the central curve from round bar and flattening it was fairly easy, but heating the ends of the curve enough to give a tight bend to the bar was an exercise in frustration. With a shallower forge this could have been accomplished much more easily, because the correct area of the little arch could have been trivially heated to the correct temperature. Modern gas-welding equipment would have also done the trick nicely. The total weight increase caused by the new little ladder and new little arch (~750 grams) will be more than compensated for by the new case, which is made from good quality pine, and which will weight about 4 kilos when finished. The old birch case weighs a lot more, around 5.5 kilos. The final problem I had to solve was related to rotating the washers. With 250hz pretensioning the washers could be rotated with fairly little effort. With 320hz I started having problems, because my proof of concept plywood washer rotation tool started breaking up. As the torsion springs are going to be tuned to 350hz, the plywood contraption was simply too weak, as well as having too little leverage. So I decided to make a longer tool from steel. After pondering the design a bit, I ended up making it an exercise in simplicity: I have not yet tested the tool in practice, but I don't see why it would not work. Category:Blog posts Category:Backup Category:Little ladder Category:Little arch Category:Practical Category:Metalworking Category:Cheiroballistra Category:Washer